Hybrid multi-level power converter with inter-stage inductor

ABSTRACT

The present document relates to a power converter comprising an inductor, a first stage, and a second stage. The first stage may be coupled between an input of the power converter and the inductor, and the first stage may comprise a first flying capacitor. The second stage may be coupled between the inductor and an output of the power converter, and the second stage may comprise a second flying capacitor. A second terminal of the first flying capacitor may be connected to a first terminal of the inductor, and a first terminal of the second flying capacitor may be connected to a second terminal of the inductor.

TECHNICAL FIELD

The present document relates to power converters. In particular, thepresent document relates to hybrid multi-level power converters withimproved efficiency.

BACKGROUND

There is an increasing demand for high-efficiency, regulated powerconverters in several market segments such as e.g. solid-state drives(SSDs), computing device, charging devices, or wearable power managementintegrated circuits (PMICs). Specifically, it is highly desirable todesign power converters with higher efficiency and smaller area thanconventional buck converters.

SUMMARY

Within this document, hybrid multi-level power converters are proposed.At this, the term “multi-level” indicates that the power convertercomprises an inductor, and that the power converter is capable ofapplying more than two different voltages across this inductor.Moreover, the present document concerns “hybrid” power converters, i.e.power converters comprising both flying capacitors and inductors. Withregard to the overall area of the power converter, the sizes of therequired inductors and flying capacitors are important designparameters. Thus, it is important to reduce the sizes of the lattercircuit elements.

Moreover, there is a demand for power converters that perform powerconversion with a small input-to-output voltage conversion ratioV_(OUT)/V_(IN), in particular when V_(OUT)/V_(IN)<¼. As a furtherobjective, it is advantageous to the converter efficiency to limit thevoltage across the inductor during the operation of the power converter.

The present document addresses the above-mentioned technical problems.

According to an aspect, a buck power converter is presented. The buckpower converter may comprise an inductor, a first stage, and a secondstage. The first stage may be coupled between an input of the buck powerconverter and the inductor, and the first stage may comprise a firstflying capacitor. The second stage may be coupled between the inductorand an output of the buck power converter, and the second stage maycomprise a second flying capacitor.

The buck power converter may be configured to regulate an output voltageat the output of the buck power converter which is smaller than an inputvoltage at the input of the buck power converter. To this end, the buckpower converter may comprise a plurality of switching elements forcontrolling the currents flowing through the components of the buckpower converter. Further, the buck power converter may comprise afeedback loop for comparing said output voltage (or an output current)against a reference value and generating respective control signals forcontrolling the plurality of switching elements.

In general, the first and second flying capacitors may be passiveelectronic components capable of storing electrical energy in anelectric field. The capacitors may have different capacitances or mayhave the same capacitance. Each capacitor may comprise a first terminaland a second terminal. The term ‘flying’ may come from the fact that oneterminal of the capacitor is connected to different reference voltagesduring the converter operation. Therefore, it is like the chargedcapacitor flies from a voltage to another.

A second terminal of the first flying capacitor may be connected to afirst terminal of the inductor, and a first terminal of the secondflying capacitor may be connected to a second terminal of the inductor.In other words, the first flying capacitor, the inductor, and the secondflying capacitor may form a serial connection between the input of thebuck power converter and the output of the buck power converter.

The first stage may comprise a reservoir capacitor, and the buck powerconverter may be configured to alternately couple a first terminal ofthe reservoir capacitor to the second terminal of the first flyingcapacitor (which may be connected to the inductor), or to a firstterminal of the first flying capacitor. As a second terminal of thereservoir capacitor may be connected to a reference potential (such ase.g. ground), the reservoir capacitor may discharge and contribute to acurrent flowing though the inductor in the first case, whereas, in thesecond case, the first flying capacitor may charge the reservoircapacitor.

The first stage may comprise a first switching element coupled betweenthe input of the buck power converter and the first terminal of thefirst flying capacitor. The first stage may comprise a second switchingelement coupled between the first terminal of the first flying capacitorand an intermediate node. The first stage may comprise a third switchingelement coupled between the intermediate node and the second terminal ofthe first flying capacitor. The first stage may comprise a fourthswitching element coupled between the second terminal of the firstflying capacitor and a reference potential. The reservoir capacitor maybe coupled between the intermediate node and the reference potential. Inparticular, the first terminal of the reservoir capacitor may beconnected to the intermediate node, and the second terminal of thereservoir capacitor may be connected to the reference potential.

Throughout this document, the term “reference potential” is meant in itsbroadest possible sense. In particular, the reference potential is notlimited to ground i.e. a reference potential with a direct physicalconnection to earth. Rather, the term “reference potential” may refer toany reference point to which and from which electrical currents may flowor from which voltages may be measured. Moreover, it should be mentionedthat the reference potentials mentioned in this document may notnecessarily refer to the same physical contact. Instead, the referencepotentials mentioned in this document may relate to different physicalcontacts although reference is sometimes made to “the” referencepotential for ease of presentation.

Each switching element may be implemented with any suitable device, suchas, for example, a metal-oxide-semiconductor field effect transistor(MOSFET), an insulated-gate bipolar transistor (IGBT), a MOS-gatedthyristor, or other suitable power devices. Each switching element mayhave a gate to which a driving voltage or control signal may be appliedto turn the respective switching element on (i.e. to close the switchingelement) or turn the respective switching element off (i.e. to open theswitching element).

The second stage may comprise a fifth switching element coupled betweenthe first terminal of the second flying capacitor and the output of thebuck power converter. The second stage may comprise a sixth switchingelement coupled between the output of the buck power converter and asecond terminal of the second flying capacitor. The second stage maycomprise a seventh switching element coupled between the second terminalof the second flying capacitor and a reference potential.

The buck power converter may be configured to establish, in amagnetizing state, a first magnetizing current path from the input ofthe buck power converter, via the first flying capacitor, via theinductor, and via the second flying capacitor to the output of the buckpower converter. The buck power converter may be configured toestablish, in the magnetizing state, a second magnetizing current pathfrom the reservoir capacitor, via the inductor, and via the secondflying capacitor to the output of the buck power converter. For thispurpose, the buck power converter may be configured, in the magnetizingstate, to close the first switching element, the third switchingelement, and the sixth switching element, and to open the remainingswitching elements.

The buck power converter may be configured to establish, in ademagnetizing state, a demagnetizing current path from a referencepotential, via the inductor to the output of the buck power converter.The buck power converter may be configured to establish, in thedemagnetizing state, a first current path from a reference potential,via the first flying capacitor, via the reservoir capacitor, to thereference potential, and a second current path from the referencepotential, via the second flying capacitor, to the output of the buckpower converter. For this purpose, the buck power converter may beconfigured, in the demagnetizing state, to close the second switchingelement, the fourth switching element, the fifth switching element andthe seventh switching element, and to open the remaining switchingelements. The buck power converter may comprise an output capacitorconnected between the output of the buck power converter and thereference potential.

The power converter may be configured to switch between the magnetizingstate and the demagnetizing state and control the switching elementsaccordingly.

The skilled person will readily understand that the adjective“magnetizing” is used here to describe the purpose of the magnetizingstate and magnetizing current path, respectively. While in themagnetizing state, a current may flow along the magnetizing current pathand may increase, resulting in an increase of the magnetic energy storedin the inductor. In other words, the inductor may be said to be“magnetized” during the magnetizing state. Analogously, the skilledperson will readily understand that the adjective “demagnetizing” isused here to describe the purpose of the demagnetizing state anddemagnetizing current path, respectively. While in the demagnetizingstate, a current may flow along the demagnetizing current path and maydecrease, resulting in a decrease of the magnetic energy stored in theinductor. In other words, the inductor may be said to be “demagnetized”during the demagnetizing state.

The buck power converter may be configured to establish, in analternative magnetizing state, a first magnetizing current path from theinput of the buck power converter, via the first flying capacitor, andvia the inductor to the output of the buck power converter. The buckpower converter may be configured to establish, in the alternativemagnetizing state, a second magnetizing current path from the reservoircapacitor, and via the inductor, to the output of the buck powerconverter. The buck power converter may be configured to establish, inthe alternative magnetizing state, a third current path from a referencepotential, via the second flying capacitor, to the output of the buckpower converter.

The buck power converter may be configured to establish, in analternative demagnetizing state, a demagnetizing current path from areference potential, via the inductor, and via the second flyingcapacitor to the output of the buck power converter. The buck powerconverter may be configured to establish, in the alternativedemagnetizing state, a current path from a reference potential, via thefirst flying capacitor, via the reservoir capacitor, to the referencepotential.

The power converter may be configured to switch between the magnetizingstate, the alternative magnetizing state, the demagnetizing state, andthe alternative demagnetizing state, and control the switching elementsaccordingly.

According to another aspect, a boost power converter is presented. Theboost power converter may comprise an inductor, a first stage coupledbetween an input of the boost power converter and the inductor, whereinthe first stage comprises a first flying capacitor, and a second stagecoupled between the inductor and an output of the boost power converter,wherein the second stage comprises a second flying capacitor.

The boost power converter may be configured to regulate an outputvoltage at the output of the boost power converter which is greater thanan input voltage at the input of the boost power converter. To this end,the boost power converter may comprise a plurality of switching elementsfor controlling the currents flowing through the components of the boostpower converter. Further, the boost power converter may comprise afeedback loop for comparing said output voltage (or an output current)against a reference value and generating respective control signals forcontrolling the plurality of switching elements.

A second terminal of the first flying capacitor may be connected to afirst terminal of the inductor, and a first terminal of the secondflying capacitor may be connected to a second terminal of the inductor.

The second stage may further comprise a reservoir capacitor, and theboost power converter may be configured to alternately couple a firstterminal of the reservoir capacitor to the first terminal of the secondflying capacitor, or to a second terminal of the second flyingcapacitor.

The second stage may comprise a first switching element coupled betweenthe output of the boost power converter and the second terminal of thesecond flying capacitor. The second stage may comprise a secondswitching element coupled between the second terminal of the secondflying capacitor and an intermediate node. The second stage may comprisea third switching element coupled between the intermediate node and thefirst terminal of the second flying capacitor. The second stage maycomprise a fourth switching element coupled between the first terminalof the second flying capacitor and a reference potential, wherein thereservoir capacitor is coupled between said intermediate node and thereference potential.

The first stage may comprise a fifth switching element coupled betweenthe second terminal of the first flying capacitor and the input of theboost power converter. The first stage may comprise a sixth switchingelement coupled between the input of the boost power converter and afirst terminal of the first flying capacitor. The first stage maycomprise a seventh switching element coupled between the first terminalof the first flying capacitor and a reference potential.

The boost power converter may be configured to establish, in ademagnetizing state, a first demagnetizing current path from the inputof the boost power converter, via the first flying capacitor, via theinductor, and via the second flying capacitor to the output of the boostpower converter. The boost power converter may be configured toestablish, in the demagnetizing state, a second demagnetizing currentpath from the input of the boost power converter, via the first flyingcapacitor, via the inductor, and via the reservoir capacitor, to areference potential.

The boost power converter may be configured to establish, in amagnetizing state, a magnetizing current path from the input of theboost power converter, via the inductor, to a reference potential. Theboost power converter may be further configured to establish, in themagnetizing state, a first current path from a reference potential, viathe reservoir capacitor, via the second flying capacitor, to thereference potential. The boost power converter may be further configuredto establish, in the magnetizing state, a second current path from theinput of the boost power converter, via the first flying capacitor, tothe reference potential.

The boost power converter may be configured to establish, in analternative demagnetizing state, a first demagnetizing current path fromthe input of the boost power converter, via the inductor, and via thesecond flying capacitor to the output of the boost power converter. Theboost power converter may be configured to establish, in the alternativedemagnetizing state, a second demagnetizing current path from the inputof the boost power converter, via the inductor, and via the reservoircapacitor, to a reference potential. Moreover, the boost power convertermay be configured to establish, in the alternative demagnetizing state,a current path from the input of the boost power converter, via thefirst flying capacitor to a reference potential.

The boost power converter may be configured to establish, in analternative magnetizing state, a magnetizing current path from the inputof the boost power converter, via the first flying capacitor, via theinductor, to a reference potential. The boost power converter may befurther configured to establish, in the alternative magnetizing state, acurrent path from a reference potential, via the reservoir capacitor,via the second flying capacitor, to the reference potential.

According to another aspect, a method for operating a buck powerconverter is described. The method may comprise steps which correspondto the features of the buck power converter described in the presentdocument. The buck power converter may comprise an inductor, a firststage with a first flying capacitor, and a second stage with a secondflying capacitor. The method may comprise coupling the first stagebetween an input of the buck power converter and the inductor. Themethod may comprise coupling the second stage between the inductor andan output of the buck power converter.

The first stage may further comprise a reservoir capacitor. The methodmay comprise alternately coupling a first terminal of the reservoircapacitor to a first terminal of the first flying capacitor, or to asecond terminal of the first flying capacitor.

The method may comprise establishing, in a magnetizing state, a firstmagnetizing current path from the input of the buck power converter, viathe first flying capacitor, via the inductor, and via the second flyingcapacitor to the output of the buck power converter. The method maycomprise establishing, in the magnetizing state, a second magnetizingcurrent path from the reservoir capacitor, via the inductor, and via thesecond flying capacitor to the output of the buck power converter.

The method may comprise establishing, in a demagnetizing state, ademagnetizing current path from a reference potential, via the inductorto the output of the buck power converter. The method may compriseestablishing, in the demagnetizing state, a first current path from areference potential, via the first flying capacitor, via the reservoircapacitor, to the reference potential. The method may compriseestablishing, in the demagnetizing state, a second current path from thereference potential, via the second flying capacitor, to the output ofthe buck power converter.

According to yet another aspect, a method of operating a boost powerconverter is presented. The method may comprise steps which correspondto the features of the boost power converter described in the presentdocument. The boost power converter may comprise an inductor, a firststage with a first flying capacitor, and a second stage with a secondflying capacitor. The method may comprise coupling the first stagebetween an input of the boost power converter and the inductor. Themethod may comprise coupling the second stage between the inductor andan output of the boost power converter.

The second stage may further comprise a reservoir capacitor, and themethod may further comprise alternately coupling a first terminal of thereservoir capacitor to the first terminal of the second flyingcapacitor, or to a second terminal of the second flying capacitor.

The method may comprise establishing, in a demagnetizing state, a firstdemagnetizing current path from the input of the boost power converter,via the first flying capacitor, via the inductor, and via the secondflying capacitor to the output of the boost power converter. The methodmay comprise establishing, in the demagnetizing state, a seconddemagnetizing current path from the input of the boost power converter,via the first flying capacitor, via the inductor, and via the reservoircapacitor to a reference potential.

The method may comprise establishing, in a magnetizing state, amagnetizing current path from the input of the boost power converter,via the inductor, to a reference potential. The method may compriseestablishing, in the magnetizing state, a first current path from areference potential, via the reservoir capacitor, via the second flyingcapacitor, to the reference potential. The method may compriseestablishing, in the magnetizing state, a second current path from theinput of the boost power converter, via the first flying capacitor, tothe reference potential.

The method may comprise establishing, in an alternative demagnetizingstate, a first demagnetizing current path from the input of the boostpower converter, via the inductor, and via the second flying capacitorto the output of the boost power converter. The method may compriseestablishing, in the alternative demagnetizing state, a seconddemagnetizing current path from the input of the boost power converter,via the inductor, and via the reservoir capacitor, to a referencepotential. Moreover, the method may comprise establishing, in thealternative demagnetizing state, a current path from the input of theboost power converter, via the first flying capacitor to a referencepotential.

The method may comprise establishing, in an alternative magnetizingstate, a magnetizing current path from the input of the boost powerconverter, via the first flying capacitor, via the inductor, to areference potential. The method may comprise establishing, in thealternative magnetizing state, a current path from a referencepotential, via the reservoir capacitor, via the second flying capacitor,to the reference potential.

According to a further aspect, a software program is described. Thesoftware program may be adapted for execution on a processor and forperforming the method steps outlined in the present document whencarried out by the processor.

According to another aspect, a storage medium is described. The storagemedium may comprise a software program adapted for execution on aprocessor and for performing the method steps outlined in the presentdocument when carried out by the processor.

According to a further aspect, a computer program product is described.The computer program product may comprise instructions for performingthe method steps outlined in the present document when carried out bythe processor.

It should be noted that the methods and systems including its preferredembodiments as outlined in the present document may be used stand-aloneor in combination with the other methods and systems disclosed in thisdocument. In addition, the features outlined in the context of a systemare also applicable to a corresponding method. Furthermore, all aspectsof the methods and systems outlined in the present document may bearbitrarily combined. In particular, the features of the claims may becombined with one another in an arbitrary manner.

In the present document, the term “couple” or “coupled” refers toelements being in electrical communication with each other, whetherdirectly connected e.g., via wires, or in some other manner.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and not by wayof limitation, in the figures of the accompanying drawings in which likereference numerals refer to similar or identical elements, and in which

FIG. 1 shows an exemplary power converter;

FIG. 2 shows current paths when the exemplary power converter isoperated in a buck mode in a magnetizing state;

FIG. 3 shows current paths when the exemplary power converter isoperated in a buck mode in a demagnetizing state;

FIG. 4 shows inductor current ripple for different conversion ratios fora conventional buck power converter and for the proposed multi-levelhybrid power converter operated in buck mode;

FIG. 5 shows current paths when the exemplary power converter isoperated in a buck mode in an alternative magnetizing state;

FIG. 6 shows current paths when the exemplary power converter isoperated in a buck mode in an alternative demagnetizing state;

FIG. 7 shows current paths when the exemplary power converter isoperated in a boost mode in a magnetizing state;

FIG. 8 shows current paths when the exemplary power converter isoperated in a boost mode in a demagnetizing state;

FIG. 9 shows current paths when the exemplary power converter isoperated in a boost mode in an alternative magnetizing state;

FIG. 10 shows current paths when the exemplary power converter isoperated in a boost mode in an alternative demagnetizing state.

FIG. 11 shows a method for operating a buck power converter.

DETAILED DESCRIPTION

FIG. 1 shows an exemplary power converter 100 according to the teachingsof the present document. The exemplary power converter 100 comprises aninductor 1, a first stage, and a second stage. The first stage iscoupled between an input of the power converter 100 and the inductor 1,capacitor 2 is connected to a first terminal of the inductor 1, and afirst terminal of the second flying capacitor 3 is connected to a secondterminal of the inductor 1.

The first stage of the exemplary power converter 100 comprises a firstswitching element 11 coupled between the input of the power converterand the first terminal of the first flying capacitor 2. It furthercomprises a second switching element 12 coupled between the firstterminal of the first flying capacitor 2 and an intermediate node. Thepower converter 100 further comprises a third switching element 13coupled between the intermediate node and the second terminal of thefirst flying capacitor 2, and a fourth switching element 14 coupledbetween the second terminal of the first flying capacitor 2 and areference potential. A reservoir capacitor 4 is coupled between theintermediate node and the reference potential. The power converter 100controls the second switching element 12 and the third switching element13 such that a first terminal of the reservoir capacitor 4 isalternately coupled to (a) the second terminal of the first flyingcapacitor 2, or (b) to a first terminal of the first flying capacitor 2.In other words, in the first stage, the first flying capacitor C_(F1)and the reservoir capacitor C_(R) are alternately placed in series andin parallel, and therefore develop a voltage of V_(IN)/2 across them,wherein V_(IN) denotes the input voltage at the input of power converter100.

As illustrated in FIG. 1, the second stage comprises a fifth switchingelement 15 coupled between the first terminal of the second flyingcapacitor 3 and the output of the power converter, a sixth switchingelement 16 coupled between the output of the power converter and asecond terminal of the second flying capacitor 3, and a seventhswitching element 17 coupled between the second terminal of the secondflying capacitor 3 and a reference potential. Moreover, an outputcapacitor 5 is typically connected between the output of the powerconverter and the reference potential. In the second stage, the secondflying capacitor 3 is periodically connected to the output voltageV_(OUT) of the power converter, therefore determining its averagevoltage.

The proposed power converter 100 may be operated in a forward directiondenoted as buck mode, in which power converter 100 regulates an outputvoltage at the output of the buck power converter which is smaller thanan input voltage at the input of the buck power converter. The buck modeis illustrated using respective current flows in FIGS. 2, 3, 5, and 6.

In the buck mode, the proposed power converter 100 may operate with onlytwo states without the need of regulating the voltage of its flyingcapacitors 2 and 3. Indeed, the voltages across the flying capacitorsare inherently determined by the power converter operation as will bediscussed in the following description. For example, the power convertermay switch between a magnetizing state and a demagnetizing state.

FIG. 2 shows current paths (illustrated by respective arrows) when theexemplary power converter is operated in a buck mode in a magnetizingstate D1. During the magnetizing state D1, the inductor is magnetized.The voltage of node X is V_(IN)/2. The first 11 and the third 13switching element are closed, and the second 12 and the fourth 14switching element are open. Switching element 16 is closed and switchingelement 15 and 17 are open. Node Y is at a voltage 2V_(OUT). Theinductor current is sourced by both the input voltage V_(IN) via thefirst flying capacitor C_(F1) (at a voltage of V_(IN)/2) and by thereservoir capacitor C_(R) (charged at V_(IN)/2). The second flyingcapacitor C_(F2) (at a voltage of V_(OUT)) decouples node Y from theoutput of the power converter.

In other words, the first flying capacitor and the second flyingcapacitor are placed in series between V_(IN) and the referencepotential during D1, and in parallel during demagnetizing state D2.

FIG. 3 shows current paths when the exemplary power converter isoperated in a buck mode in a demagnetizing state D2. During thedemagnetizing state D2, the inductor is demagnetized by connecting nodeX to the reference potential and node Y to V_(OUT). The first flyingcapacitor C_(F1) is placed in parallel with the reservoir capacitorC_(R) since S1 and S3 are open, and S2 and S4 are closed. The secondflying capacitor C_(F2) in parallel with the output capacitor C_(OUT).S5 and S7 are closed, and S6 is open.

The charge balance of the flying and reservoir capacitors is guaranteedduring the converter operation as the capacitors experience currentflows with opposite directions during the two states D1 and D2. Duringthe magnetizing state D1, the first flying capacitor C_(F1) and thesecond flying capacitor C_(F2) charge, while the reservoir capacitorC_(R) discharges. During the demagnetizing state D2, C_(F1) and C_(F2)discharge, while C_(R) charges. The relationship between input andoutput voltage is obtained by applying the volt-second balance principle(with D1=D and D2=1−D) to the inductor voltage v_(L):

$\begin{matrix}{\frac{V_{OUT}}{V_{IN}} = {{\frac{0.5 \cdot D}{1 + D}D} \in \left\lbrack {0,1} \right\rbrack}} & (1)\end{matrix}$

The maximum theoretical input-to-output conversion ratio V_(OUT)/V_(IN)is ¼ for D=1. FIG. 4 shows inductor current ripple ΔI_(L) for differentconversion ratios for a conventional buck power converter 410(0<V_(OUT)/V_(IN)<1) and for the proposed multi-level hybrid powerconverter operated in buck mode 420 (0<V_(OUT)/V_(IN)<0.25). Because ofthe reduced inductor current ripple ΔI_(L), inductor core losses aresignificantly reduced for 0<V_(OUT)/V_(IN)<0.25.

Since the inductor is placed between the two stages, the averageinductor current is reduced by the voltage conversion ratio of thesecond stage compared to topologies that use the inductor at theconverter output. Therefore, for a given inductor, DCR losses arereduced by the square of the voltage conversion ratio of the secondstage. The reduced current rating for the inductor also allows to reduceits physical dimensions.

Let us now examine the voltage rating of the FETs in the new converter.Devices with lower voltage rating have typically a better Figure ofMerit (smaller specific resistance and smaller gate capacitance). Insteady state conditions, the voltage rating for the devices of the newhybrid converter is:

-   -   V_(IN)/2 for S1, S2, S3, S4    -   V_(OUT) for S5, S6, S7

The voltage rating is reduced compared to that of a conventional buckconverter (requiring V_(IN)-rated devices).

The operation of the power converter can be improved by allowing theflying capacitor of the second stage C_(F2) to further provide part ofthe load current. This can be achieved by introducing two additionaloperation states DP and DV. FIG. 5 shows current paths when theexemplary power converter is operated in a buck mode in an alternativemagnetizing state DP. FIG. 6 shows current paths when the exemplarypower converter is operated in a buck mode in an alternativedemagnetizing state DV.

During state DP, C_(F2) is discharged to the output also during themagnetizing phase of the inductor (in the two-phase operation of FIGS. 3and 4, this is limited to the demagnetizing state D2). During state DV,the charge on C_(F2) is balanced by charging it. The charge balance isalso guaranteed for C_(F1) and C_(R) as in the previously describedtwo-phase operation of FIGS. 3 and 4.

In addition, phase DP can be used to reduce the drop of the outputvoltage when a transient load current has been applied. In a similar wayoperation phase DV can be inserted into the switching sequence to reduceoutput voltage overshoot, when load current is suddenly removed.

The duration of the phases DP and DV can be chosen in a way that limitsthe losses (non-adiabatic and conduction) due to the discharge currentgoing from C_(F2) to C_(OUT).

Therefore, the magnetizing phase of the inductor can be split betweenstates D1 and DP, while the demagnetizing phase can be split betweenstates D2 and DV. The input-to-output voltage conversion ratio for twodifferent example allocations of phase durations is reported below:

$\begin{matrix}{{\frac{V_{OUT}}{V_{IN}} = {{\frac{3 \cdot D}{10}D1} = D}},{{DP} = 0},{{D2} = \frac{1}{3}},{{DV} = {{\frac{2}{3} - {DD}} \in \left\lbrack {0,\frac{2}{3}} \right\rbrack}}} & (2)\end{matrix}$ $\begin{matrix}{{\frac{V_{OUT}}{V_{IN}} = {{\frac{D}{3}D1} = {{DP} = \frac{D}{2}}}},{{D2} = {{DV} = {{\frac{1 - D}{2}D} \in \left\lbrack {0,1} \right\rbrack}}}} & (3)\end{matrix}$

Therefore, in (2) and (3) the flying capacitor C_(F2) is connected toC_(OUT) for a fixed duration (during DP and D2): ⅓ and ½ of theswitching period, respectively. The corresponding charging phase forC_(F2) (during D1 and DV) is ⅔ or ½ of the switching period for cases(2) and (3), respectively. Therefore, the average C_(F2) dischargecurrent is twice or equal to the corresponding charge current in cases(2) and (3), respectively. This results in reduced peak current inswitch S5, reduced (40% and 33%, respectively) average inductor current,and a different theoretical maximum input-to-output conversion ratioV_(OUT)/V_(IN) (⅕ or ⅓, respectively). It should be noted that anoperation with D close to 1 may only be considered theoretically, as thetime interval for charge balancing of C_(F1) (and C_(F2)) thenapproaches zero. From that the real-world maximum input-to-outputconversion ratio typically remains V_(OUT)/V_(IN)<¼.

As already mentioned, the proposed power converter may be denoted as“multi-level” power converter, indicating that the power converter iscapable of generating more than two different voltages across thisinductor. Specifically, the voltages across the inductor are:

-   -   V_(IN/)2−2V_(OUT) during D1    -   −V_(OUT) during D2    -   V_(IN)/2−V_(OUT) during DP    -   −2V_(OUT) during DV.

The two converter stages may be integrated into a single converter unit,but alternatively also implemented inside separate units. As an example,switches S1-S4, the flying capacitor C_(F1) and the inductor L couldbecome part of a pre-converter stage generating a PWM signal with anaverage level close to the required bus voltage. Its operation may belimited to compensate the variations of the converter input voltage(line regulation). Switches S5-S7 and the flying capacitor C_(F2),instead, can serve as a second stage that adapts its duty cycle tofine-tune the overall converter output voltage, i.e. handling the dropcaused by variable load current (load regulation). The 2^(nd) stage mayeven be combined with the POL (Point of Load), i.e. go into the packageof a microprocessor. This is supported by the fact that capacitors canstore ˜1000 times the energy of an inductor with the same volume, and bythe fact that the switches need to handle only voltages in the range ofthe input voltage of the load. Therefore, the second stage couldeventually become part of the microprocessor IC itself.

Finally, by inverting the roles of input and output ports the describedtopology can be used to perform an efficient boost power conversion withlarge conversion ratio. The magnetizing (de-magnetizing) phase in thebuck operation becomes a de-magnetizing (magnetizing) phase in the boostoperation. FIG. 7 shows current paths when the exemplary power converteris operated in a boost mode in a magnetizing state. FIG. 8 shows currentpaths when the exemplary power converter is operated in a boost mode ina demagnetizing state. The voltage on the flying capacitors C_(F1) andC_(F2) is V_(OUT)/2 and V_(IN), respectively. During D1, the inductor isdemagnetized and C_(F1) and C_(F2) discharge, while C_(R) charges.During D2, C_(F1) and C_(F2) charge, while C_(R) discharges. As for thebuck operation, also for the boost operation it is possible to introducetwo extra states as shown in FIGS. 9 and 10.

FIG. 11 shows 1100, a method for operating a buck power converter. Thesteps include 1110, providing a buck power converter, comprising aninductor, a first stage with a first flying capacitor, and a secondstage with a second flying capacitor. The steps also include 1120,coupling the first stage between an input of the buck power converterand the inductor, and 1130, coupling the second stage between theinductor and an output of the buck power converter.

In summary, the proposed power converter offers the advantage thatflying capacitor regulation is inherently achieved by the topology. Thatis, the voltage across the capacitors is defined by Kirchhoff VoltageLaw (KVL) during the operation of the converter. Furthermore, theproposed power converter offers the following advantages: reducedvoltage rating for MOSFET devices for the same input and outputvoltages, reduced losses associated to the inductor DCR (i.e. the directcurrent parasitic resistance DCR of the inductor) for the same outputcurrent, reduced current rating (thermal and saturation limit) forinductor, improved transient load response, and reduced peak inputcurrent ripple.

It should be noted that the description and drawings merely illustratethe principles of the proposed methods and systems. Those skilled in theart will be able to implement various arrangements that, although notexplicitly described or shown herein, embody the principles of theinvention and are included within its spirit and scope. Furthermore, allexamples and embodiment outlined in the present document are principallyintended expressly to be only for explanatory purposes to help thereader in understanding the principles of the proposed methods andsystems. Furthermore, all statements herein providing principles,aspects, and embodiments of the invention, as well as specific examplesthereof, are intended to encompass equivalents thereof.

What is claimed is:
 1. A buck power converter comprising an inductor, afirst stage coupled between an input of the buck power converter and theinductor, wherein the first stage comprises a first flying capacitor,and a second stage coupled between the inductor and an output of thebuck power converter, wherein the second stage comprises a second flyingcapacitor, wherein a second terminal of the first flying capacitor isconnected to a first terminal of the inductor, and a first terminal ofthe second flying capacitor is connected to a second terminal of theinductor, wherein the first stage further comprises a reservoircapacitor, and the buck power converter is configured to alternatelycouple a first terminal of the reservoir capacitor: to the secondterminal of the first flying capacitor, or to a first terminal of thefirst flying capacitor, wherein the buck power converter is configuredto establish, in a magnetizing state, a first magnetizing current pathfrom the input of the buck power converter, via the first flyingcapacitor, via the inductor, and via the second flying capacitor to theoutput of the buck power converter, and—a second magnetizing currentpath from the reservoir capacitor, via the inductor, and via the secondflying capacitor to the output of the buck power converter; and/orwherein the buck power converter is configured to establish, in ademagnetizing state, a demagnetizing current path from a referencepotential, via the inductor to the output of the buck power converter, afirst current path from a reference potential, via the first flyingcapacitor, via the reservoir capacitor, to the reference potential, anda second current path from the reference potential, via the secondflying capacitor, to the output of the buck power converter.
 2. The buckpower converter of claim 1, wherein the first stage comprises a firstswitching element coupled between the input of the buck power converterand the first terminal of the first flying capacitor, a second switchingelement coupled between the first terminal of the first flying capacitorand an intermediate node, a third switching element coupled between theintermediate node and the second terminal of the first flying capacitor,and a fourth switching element coupled between the second terminal ofthe first flying capacitor and a reference potential, wherein thereservoir capacitor is coupled between the intermediate node and thereference potential.
 3. The buck power converter of claim 1, wherein thesecond stage comprises a fifth switching element coupled between thefirst terminal of the second flying capacitor and the output of the buckpower converter, a sixth switching element coupled between the output ofthe buck power converter and a second terminal of the second flyingcapacitor, and a seventh switching element coupled between the secondterminal of the second flying capacitor and a reference potential.
 4. Abuck power converter comprising an inductor, a first stage coupledbetween an input of the buck power converter and the inductor, whereinthe first stage comprises a first flying capacitor, and a second stagecoupled between the inductor and an output of the buck power converter,wherein the second stage comprises a second flying capacitor, wherein asecond terminal of the first flying capacitor is connected to a firstterminal of the inductor, and a first terminal of the second flyingcapacitor is connected to a second terminal of the inductor, wherein thefirst stage further comprises a reservoir capacitor, and the buck powerconverter is configured to alternately couple a first terminal of thereservoir capacitor: to the second terminal of the first flyingcapacitor, or to a first terminal of the first flying capacitor, whereinthe buck power converter is configured to establish, in an alternativemagnetizing state, a first magnetizing current path from the input ofthe buck power converter, via the first flying capacitor, and via theinductor to the output of the buck power converter, a second magnetizingcurrent path from the reservoir capacitor, and via the inductor, to theoutput of the buck power converter, and a third current path from areference potential, via the second flying capacitor, to the output ofthe buck power converter, and/or wherein the buck power converter isconfigured to establish, in an alternative demagnetizing state, ademagnetizing current path from a reference potential, via the inductor,and via the second flying capacitor to the output of the buck powerconverter, and a current path from a reference potential, via the firstflying capacitor, via the reservoir capacitor, to the referencepotential.
 5. A method of operating a buck power converter comprising aninductor, a first stage with a first flying capacitor, and a secondstage with a second flying capacitor, the method comprising coupling thefirst stage between an input of the buck power converter and theinductor, and coupling the second stage between the inductor and anoutput of the buck power converter, wherein the first stage furthercomprises a reservoir capacitor, and the method comprises alternatelycoupling a first terminal of the reservoir capacitor to a first terminalof the first flying capacitor, or to a second terminal of the firstflying capacitor, wherein the method comprises establishing, in amagnetizing state, a first magnetizing current path from the input ofthe buck power converter, via the first flying capacitor, via theinductor, and via the second flying capacitor to the output of the buckpower converter, and a second magnetizing current path from thereservoir capacitor, via the inductor, and via the second flyingcapacitor to the output of the buck power converter, and/or wherein themethod comprises establishing, in a demagnetizing state, a demagnetizingcurrent path from a reference potential, via the inductor to the outputof the buck power converter a first current path from a referencepotential, via the first flying capacitor, via the reservoir capacitor,to the reference potential, and a second current path from the referencepotential, via the second flying capacitor, to the output of the buckpower converter.